1. Field of the Invention
This invention relates to polymeric compositions suitable for use as high voltage insulating materials.
2. Summary of the Prior Art
Failure of polymeric insulating materials through tracking is caused by the formation of a dendritic carbonaceous path on the surface of material. It is known to incorporate anti-tracking additives into polymers, and it has been found that alumina hydrates, especially alumina trihydrate, Al.sub.2 O.sub.3.3H.sub.2 O, are particularly effective. Other additives that have been proposed include alumina, magnesia and magnesia hydrates. However, polymeric compositions containing anti-tracking additives suffer from the disadvantage that when subjected to high voltage stress, they fail as a result of progressive erosion of the material. In addition, effective amounts of anti-tracking additives are somewhat higher than is desirable, e.g. 50 to 90% of the composition (percentages are by weight throughout this specification), particularly when the composition contains a hydrated additive and is subjected to conditions which cause loss of water of hydration, e.g. during extrusion, moulding or cross-linking. One solution to the erosion problem, as described in British Pat. Nos. 1337951 and 1337952 and U.S. application Ser. No. 434,126 (the disclosures of which are hereby incorporated by reference), is to incorporate in the composition an oxide of an element of the transition series, lanthanide series or non-transuranic actinide series, e.g. iron oxide.